Learn English – Single word for “less available” with a corresponding noun and without a cumbersome prefix

I want to find the best word for a term in a paper I am writing.

An example:

We see that Xs with higher unavailability generally have more attractive amenities.

Here, I want to replace "unavailability" with a different term.

Some details:

In my paper, I am talking about a scalar value for each of several Xs.

I want to refer to it as making the X "less available" because the higher its value, the less likely it is that the X will be any given person's "choice set" (which, you can probably guess, refers to the set of things they can choose from).

I expect to have to use the word, or variations on it, many times in writing and presentation, so I hope to avoid using an awkward negation like "unavailable". It will also be convenient to have a noun form, analogous to "unavailability" but less awkward.

A more down-to-earth example (with bad terms in square brackets):

Workers are not free to work at any firm they wish. Some firms are growing, and therefore have more openings; while others set higher standards for the quality of potential hires. These two factors cannot be separated in the data, so I combine them into [an 'unavailability index'] for each firm. From the perspective of a worker, opportunities to move to firms with a higher values in this index are [less frequently available].

Sorry if that's too long, or still too abstract. I am strongly leaning towards @WS2's suggestion, which would replace the two sets of brackets with "a measure of scarcity" and "scarcer."